Method and burner for manufacturing a glass optical fibre preform by vapour deposition

ABSTRACT

A method is disclosed for feeding a flow of gas to a burner for manufacturing an optical fibre preform, said burner comprising: a plurality of coaxial tubes, each two adjacent coaxial tubes defining an annular channel between themselves; an annular gas distribution chamber at one extremity of said annular channels and in fluid communication therewith, said annular distribution chamber being delimited in the radial direction by an inner and an outer surface; said method comprising introducing the flow of gas into the distribution chamber so that the direction of its radially outermost portion is tangential to the radially outer surface of the distribution chamber. The method allows obtaining a better gas velocity distribution in said annular channels. A burner for performing said method is also disclosed.

FIELD OF THE INVENTION

The present invention relates to a method for feeding a flow of gas to aburner for manufacturing an optical fibre preform used to make opticalglass fibres and to a burner for manufacturing said optical fibrepreform.

In particular, the present invention relates to a method for feeding aflow of gas to a burner for manufacturing an optical fibre preform, saidburner comprising a plurality of coaxial channels, in a manner thatallows to achieve a better gas velocity distribution in said channelsand to a burner for performing said method.

BACKGROUND ART

Glass fibres for optical communication are made from high purity,silica-based glass fibres drawn from glass preforms, which preforms areproduced according to various glass deposition techniques.

Some of these deposition techniques, including vapour axial deposition(VAD) and outside vapour deposition (OVD), are based on flame combustionwherein reactants (i.e. silica precursors, such as SiCl₄, optionallytogether with dopants materials, such as GeCl₄, for suitably modifyingthe refractive index of the glass) are fed together with combustinggases through a deposition burner which directs a high temperature flowof forming fine glass particles onto a rotating growing target preform.

According to the VAD deposition technique, the growth of the preformtakes place in an axial direction. Thus, the deposition burner(s) istypically maintained in a substantially fixed position, while therotating preform is slowly moved upwardly (or downwardly) with respectto the burner, in order to cause the axial growth of the preform.Alternatively, the rotating preform can be maintained in a substantiallyfixed position, while the deposition burner is slowly moved downwardly(or upwardly) with respect to the preform.

Differently from the VAD technique, in the OVD technique the growth ofthe preform takes place in a radial direction. In this case, a rotatingtarget (e.g. a quartz glass rod) is generally positioned in a fixedhorizontal or vertical position and the deposition burner is repeatedlypassed along the surface of the growing preform for causing the radialgrowth of the same.

Independently from the applied deposition technique, a porous glasspreform is thus fabricated, which is then consolidated to form a solidglass preform apt for being subsequently drawn into an optical fibre.

Typically, an optical fibre preform comprises a central portion (core)and an outer portion (cladding), the core and the cladding differing intheir respective chemical composition and having thus differentrefractive indexes. As in the optical fibres, the cladding portion formsthe majority of the preform. The preform is typically manufactured byproducing and consolidating a first preform comprising the core and afirst portion of the cladding. An overcladding layer is then depositedonto said first preform, thus obtaining a porous preform, which is thenconsolidated into the final preform.

In general, conventional burners for manufacturing optical fibrepreforms are made up of a plurality of coaxial tubes through which theglass precursor materials (i.e. silica precursors, such as SiCl₄,optionally together with dopants materials, such as GeCl₄), thecombusting gases (e.g. oxygen and hydrogen or methane) and, optionally,some inert gas (e.g. argon or helium) are fed. Typically, the glassprecursor material is fed through the central tube of the burner, whileother gases are fed through the annular channels defined by thecoaxially disposed tubes.

Generally, the gases are introduced at one extremity of each annularchannel.

U.S. Pat. No. 4,417,692 describes a burner for manufacturing an opticalfibre preform comprising a plurality of coaxial tubes defining aplurality of annular channels between each pair of adjacent tubes,having an annular chamber at the extremity of each annular channel. Thechambers are radially delimited by two cylindrical concentric surfaces.A feeding duct is connected to each chamber to feed a gas into it. Thefeeding ducts are disposed perpendicularly to the axis of the coaxialtubes; their direction thus intersects the inner surface delimiting theannular chambers.

U.S. Pat. No. 4,661,140 describes a burner for manufacturing an opticalfibre preform comprising a plurality of coaxial tubes defining aplurality of annular channels between each pair of adjacent tubes. Thegas is fed into the annular channels directly by means of pipes disposedperpendicularly to the axis of the coaxial tubes. Also in this case, thedirection of said pipes intersects the inner surface delimiting saidannular channels.

The Applicant has however observed that the disposition, of said feedingducts or pipes may not allow a completely satisfactory uniformdistribution of the gas flowing through the annular channels of theburners. For possibly optimising the gas distribution along thecircumference of said channels, the Applicant has now found a new methodfor feeding the gases into distribution chambers that are connected tosaid annular channels and a burner for implementing the said method. Ithas in fact been found that by imparting to the flow of gas fed to thedistribution chambers a direction not incident to the axis of thecoaxial tubes, in particular a direction substantially tangential to theinner surface of the distribution chamber the problem can be overcome.

Advantageously, an optimised distribution of gases according to thepresent invention may allow using deposition burners having shorterlengths. As a matter of fact, in the burners of the prior art, asubstantial length of the tubes forming the annular channels isnecessary, in order to allow the flow of gas to reach a substantialuniformity before exiting from said annular channels. According to thepresent invention, a substantially uniform flow of gas is insteadobtained within a relatively short distance from the entrance of saidgas into the annular channels. Consequently, the length of the tubesforming the annular channels can be advantageously reduced, if desired.

SUMMARY OF THE INVENTION

The Applicant has now developed a method for feeding a flow of gas intoa burner for manufacturing an optical fibre preform, said burnercomprising:

a plurality of coaxial tubes, each two adjacent coaxial tubes definingan annular channel therebetween;

an annular gas distribution chamber at one extremity of at least one ofsaid annular channels and in fluid communication therewith, said annulardistribution chamber being delimited in the radial direction by an innerand an outer surface; and

a feeding duct to feed a flow of gas into said distribution chamber,

said method comprising the step of introducing the gas into saiddistribution chamber with a direction not intersecting the axis of saidcoaxial tubes and lying on a plane intersecting said axis.

According to a preferred embodiment, said gas has a direction lying on aplane perpendicular to the axis of said coaxial tubes.

It is preferred that said flow of gas entering the distribution chamberhas a direction not intersecting the inner surface of the distributionchamber. More preferably, the direction of the radially outermostportion of the flow of gas entering said distribution chamber issubstantially tangential to the radially outer surface thereof. The term“radially” is always referred to the radial distances as measured fromthe axis of the coaxial tubes.

After having been fed to the distribution chamber, the gas leaves saiddistribution chamber and is introduced into the annular channel withwhich the distribution chamber is in fluid communication. It ispreferred that a direction substantially parallel to the axis of thecoaxial tubes is conferred to the gas flowing along said annularchannel. According to a preferred embodiment, this can be achieved bypassing the gas through a layer of porous material. Said layer may beadvantageously placed transversally to the gas flow in the said channel,or, preferably, at the entrance of the channel. More preferably, thelayer is placed between the distribution chamber and the annularchannel.

According to another aspect, the present invention relates to a burnerfor manufacturing an optical fibre preform said burner comprising:

-   -   a plurality of coaxial tubes, each two adjacent coaxial tubes        defining an annular channel between themselves    -   a gas distribution chamber at one extremity of at least one of        said annular channels and    -   a device for feeding a gas into said distribution chamber,

wherein said device imparts to the flow of said gas entering thedistribution chamber a direction not intersecting the axis of saidcoaxial tubes and lying on a plane transversal thereto.

According to a preferred embodiment, the device comprises a feeding ductconnected to said distribution chamber, said feeding duct being disposedin a direction substantially tangential with respect to the innersurface of the distribution chamber said inner surface being as abovedefined.

A layer of porous material is advantageously placed at the inlet sectionof the annular channel.

Preferably, the coaxial tubes have a circular cross-section. However,they may also have cross-sections of other shape, such as an ellipticalcross-section. The section of the distribution chamber on a planeperpendicular to the axis of the coaxial tubes has preferably the sameshape of the cross-section of said annular channels.

According to an embodiment of the invention, the lower extremities oftwo adjacent coaxial tubes defining an annular channel lie on differentplanes in particular, the innermost of the two tubes has a portionextending below the outermost one; a portion of the outer surface of theinner tube outside the annular channel may thus constitute the innersurface of the distribution chamber.

A further aspect of the present invention relates to a method formanufacturing an optical fibre preform by directing a flow of fine glassparticles from a deposition burner comprising a plurality of channelsonto a rotating elongated target preform by using a deposition burner asabove described.

The coaxial tubes can be made of any suitable material, such as metallicmaterials or quartz glass. Preferred are metallic materials, stainlesssteel being more preferred.

The porous material layer may be made of a porous metallic material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic longitudinal sectional view of a part of aburner according to the present invention;

FIGS. 2 and 3 respectively show a lateral sectional view and a top viewof a cylindrical member constituting part of the burner, as will belater described, bearing a distribution chamber.

FIG. 4 shows a bottom view of a part of a burner according to thepresent invention;

FIG. 5 shows a schematic transversal cross-sectional view of anembodiment of a burner according to the present invention;

FIG. 6 schematically shows an overcladding deposition step of a methodfor manufacturing an optical fibre preform according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically a longitudinal section of a burner accordingto one embodiment of the invention.

The burner comprises a metallic body 200 containing the lowerextremities of eight coaxial tubes, indicated with references 101 to108, and the distribution chambers 204 (only one of said chambers beingnumerically identified). The tubes 101 to 108 constitute the terminalpart of the burner itself and end in eight circular orifices placed attheir extremities opposite to those contained in the body 200. Fromthose extremities, the process gases are discharged and are then made toreact in a flame.

Eight pipes, of which only two (402, 403) are shown in the figure, areused to feed the process gases to the burner.

According to a further embodiment, the body 200 of the burner comprisesnine superimposed metallic cylindrical members 201-209.

Each of the tubes 101 to 108 is welded or otherwise firmly connected toone of the cylindrical members 201-209 in a corresponding seat. Anannular channel is defined between each pair of said tubes; the annularchannels are indicated with ref. 102 a-108 a.

In FIGS. 2 and 3 an enlarged longitudinal section and a top view of acylindrical member, indicated in FIG. 1 with ref. 204, is given. Thecylindrical member 204, specifically the fourth from the bottom end ofthe body 200 of the burner, is shown when dismantled from the burner andthe extremity of the tube 103 connected with said member 204 is alsoshown.

The member 204 presents a central bore 306, perforating it from part topart, coaxial with member 204. Coaxial with the member is also a seat304 made to receive the extremity of the tube 103 connected with member204. The bore 306 and the seat 304 may be obtained for example byboring.

In the cylindrical member, an annular distribution chamber 301 is alsoobtained. It can be appreciated from FIG. 2 that the chamber 301 isdelimited radially outwardly and downwardly by surfaces obtained in thecylindrical member 204, for example by boring. Radially inwardly, thechamber 301 is delimited by the outer wall of the tube 103 when thelatter is inserted into its seat 304. Upwardly it is delimited by alayer of porous material 302, inserted in a seat expressly obtained inthe cylindrical member 204. The porous material layer can suitably havethe form of a disk bearing a central bore to externally fit the tube103.

Suitable porous materials include multi-layered sintered metal fibres,manufactured of stainless steel, and are commercially available with aporosity ranging from 68% to 83%. An example of porous material layer,which can be advantageously used, is porous filters FIBERMET AO Series,by MEMTEC, composed by metal fibres and having a porosity of 78.4%.

A seat 303 is also obtained in the cylindrical member 204 for receivinga sealing means, for example an O-ring or a gasket.

A duct 401 is provided to connect a gas-feeding pipe 402 to thedistribution chamber 301. The end part of said duct enters the chamber301 tangentially with respect to the inner surface of the chamber, ascan be appreciated from FIG. 3. An extremity of pipe 402 enters a seatobtained in the cylindrical member 204, with which it is welded orotherwise gas-tightly connected.

Referring again to FIG. 1, cylindrical members 202, 203 and 201 areperforated so to provide an opening through the body 200 of the burnerthrough which the pipe 402 passes.

Alternatively, the superposed openings through body 200 may themselvesdefine a passage for the gas, in connection with the duct 401; sealingmeans should then be provided between each two adjacent cylindricalmembers passed through by said passage, to make it gas-tight.

The distribution chamber 301 is in connection through the porousmaterial layer 302 with the annular channel 104 a, defined between thetubes 103 and 104.

All the cylindrical members 201-209 have a structure similar to thatabove-described of member 204, except the uppermost and the lowermostmembers 201 and 209. They all present distribution chambers, ducts(analogue to the duct 401 of member 204) for connecting the distributionchambers with a gas feeding pipe (analogue to pipe 402); those structureare not shown in FIG. 1 in order to preserve the readability of thedrawing.

FIG. 4 presents a bottom view of the body 200 of a burner according tothe present invention, where the coaxial tubes, the porous materiallayers and the seats for containing the annular gaskets do not appear.More precisely, in this view every unnecessary particular has beenomitted, in order to have a clearer vision. From that view, a suitabledisposition of the said ducts (collectively indicated with ref. 400) forconnecting the distribution chambers with the said gas-feeding pipes(collectively indicated with ref. 500) can be evinced, as well as theinlet of said ducts 400 into the distribution chambers (only theradially outer surface 300 of said chambers is shown in this figure). Itcan be seen how the outer portion 405 of the ducts 400 at their entranceinto the distribution chambers, is, according to a preferred embodiment,tangential to the outer surface 305 of said chambers. Any other suitabledisposition of ducts and pipes can be applied.

The members 201-209 are assembled together as shown in FIG. 1 forexample by means of screws (not shown in FIGS. 1 to 3), which passthrough the whole body 200 through suitably obtained holes (not shown inFIGS. 1 to 3). Pins or other suitable means (not shown in FIGS. 1 to 3)are advantageously employed, according to the knowledge of the skilledin the art, to assure the precise relative positioning of said members201-209, when they are assembled.

All the disks, except top disk 209, have an annular gasket, placed in aseat analogue to the seat 303 of the member 204, to create a gas tightconnection with the adjacent disk.

According to a preferred embodiment, all the gas-feeding pipes, analogueto pipe 402, pass through the members placed below the member to whosedistribution chamber they are connected. The pipes for feeding gas tothe members above member 204, as well as the holes that permit them topass trough member 204, are not shown in FIGS. 1 to 3.

The lowermost member 201 presents a bore 404 connecting gas-feeding pipe403 to channel 101 a, which is the opening of the central tube 101. Adisk 302 of porous material can also be placed across that connection.

According to an embodiment of the invention, a tube 109, made from heatresistant material, can be disposed externally to the outer tube 108,extending for a certain length farther from the tip of said tube 108,for confining the flame. Preferably, the tube 109 extends for about 150mm to about 220 mm from the tips of the outer metal tube 108. A suitablyshaped part 110 provides a seat for said tube 109; said part 110 can beadvantageously shaped as a flanged portion of tube, as shown in FIG. 1.

The heat resistant material of tube 109 is for instance quartz glass orceramic material, such as alumina. Preferably, quartz, in particularhigh purity quartz, is employed.

According to the method to which the present invention relates, and withreference to FIG. 1, a gas is fed through pipe 402 and duct 401 to thedistribution chamber 301 of member 204, following the path indicted byarrows D and D′. As can be appreciated from FIG. 3, the gas enters thedistribution chamber 301 so that its radially outermost portion has adirection substantially tangential to the radially outer surface 305delimiting said chamber 301. The gas then flows through layer 302 to theannular channel 104 a, as shown by arrows D″, with a directionsubstantially parallel to the longitudinal axis of the burner. The gasto be flown through the other annular channels is fed in an analogueway. The gas for the central channel 101 a is preferably fed through anaxial pipe 403.

As observed by the applicant, the above described optimised distributionof the gas results in a substantially uniform axial flow of gas,achievable in a relatively short length of the annular channels, thusallowing to reduce the length of the tubes forming the burner, whendesirable.

It is preferred that the coaxial tubes are made from a metallicmaterial, more preferably from an easily machinable and heat/corrosionresistant stainless steel. An example of a suitable metal material isAISI (American Institute Steel and Iron) 316L, which is a stainlesssteel comprising about 0.03% C about 16-18% of Cr, about 11.5%-14.5% ofNi, about 2% of Mn and about 2.5%-3% of Mo.

Referring to FIG. 5, showing a cross section of the coaxial tubes in aburner according to an embodiment of the invention, reference 101 to 108being the section of said tubes, 101 a to 108 a being the channelsdefined by said tubes and 109 the section of a tube of heat resistantmaterial, designed to confine the flame as above described, the innertube 101 has typically an inner diameter of from about 6 mm to about 8mm and a thickness of from about 0.5 mm to about 2 mm.

The other tubes, having preferably a thickness comprised from about 0.5mm to about 2.5 mm, are then arranged concentrically one to each otherto form channels 102 a-108 a having widths of from about 1 mm to about3.5 mm, depending on the relative diameter of the tube and flow rate ofgas through the aperture.

In particular, the width of each channel is selected according to theamount and kind of gas that is flown through said channel and to therelative radial position of said channel. For instance, in a burnerparticularly designed for the outer cladding deposition, channelsthrough which inert gas is flown are dimensioned so to obtain an exitvelocity of the gas of from about 0.1 and about 2 m/s. Said annularchannels may thus have a width of from about 1 mm to about 1.5 mm. Onthe other side, channels through which combustion gases are flown aredimensioned so to obtain an exit velocity of the gas of from about 2 andabout 10 m/s. Said annular channels may thus have a width of from about2 mm to about 3.5 mm.

According to the preferred embodiment described above with reference toFIGS. 1 to 4, the sections perpendicular to the axis of the coaxialtubes of the distribution chambers have a circular-corona shape, theinner diameter being the diameter of the inner tube defining the annularchannel with which the distribution chamber is connected, as describedabove. The half part of the difference between the outer diameter andthe inner diameter of the said circular-corona section will be referredto as “width” of the distribution chamber. The “height” of thedistribution chamber is the distance between the two plane surfaces thataxially delimitate the distribution chamber, one of them being,according to the embodiment described in FIG. 1 to 3, the surface of theporous material layer that separates the chamber from the entrance tothe annular channel in communication therewith. The section of the ductfor feeding a gas into the distribution chamber may advantageously becircular. According to a preferred embodiment of the invention, theheight of the distribution chambers ranges from half to twice theirwidth; the height and the width of a chamber may suitably have similarvalues. Preferably, the diameter of the duct for feeding a gas into adistribution chamber is not larger than the height of the chamber;preferably, the width of the chamber is comprised between once and threetimes the diameter of said duct. The width of a distribution chamber ispreferably at least the width of the annular channel in connectiontherewith chamber; according to a preferred embodiment it is comprisedbetween once and ten times the width of said annular channel, preferablybetween once and five times. Suitably, all the distribution chambers ofa burner have the same values of height and width; the ducts for feedinggases into the distribution chambers may suitably have the samediameters as well.

Generally, through the central channel 101 a, a glass precursor materialis flown.

The flow of glass precursor material is surrounded by a flame generatedby a combusting gas and a combustion sustaining gas flowing throughfurther channels of said burner.

In the present description, the term glass precursor material isintended to refer to any suitable raw material capable of reacting inthe presence of a flame to form glass (pure silica) or doped glassparticles. Preferably, silicon tetrachloride (SiCl₄) can be used.Alternatively, other silicon containing reactants can be used, such asSiHCl₃, SiH₂Cl₂, SiH₃Cl or SiH₄. In addition, chlorine-free siliconcontaining reactants can be used, such as the siloxane compoundsdisclosed in U.S. Pat. No. 5,043,002, e.g. octamethylcyclotetrasiloxane,or the organosilicone compounds disclosed in EP A 1,016,635, e.g.hexamethyldisilane.

A preferred glass precursor material capable of forming doped glassparticles under the reaction conditions of a flame burner according tothe invention is Germanium tetrachloride. Alternative dopant materialsare POCl₃ or BBr₃.

Mixtures of the above glass precursor materials (e.g. SiCl₄ and GeCl₄)in variable proportion can be used to suitably modify the refractiveindex of the manufactured preform.

As the above glass precursor materials are generally liquid at ambienttemperature, they are preferably heated in advance into a vaporizer, sothat high temperature vapours of the glass precursor material are flownthrough the central tube of the burner. For instance, silicontetrachloride, having a boiling point of about 57° C. (at 101.330 Pa) isheated at about 100° C. in the vaporizer before being fed into theburner.

It may be advantageous, in particular for relatively large dimensionburners (e.g. cladding burners), to add a predetermined amount of a highthermal diffusivity gas to the flow of glass precursor material, inorder to increase the heat transfer from the flame towards the innercore of said flow.

The thermal diffusivity of a gas is defined as the ratio of the thermalconductivity to the heat capacity. It measures the ability of a materialto conduct thermal energy relative to its ability to store thermalenergy. Typical values of thermal diffusivity of gases can be found on anumber of reference books, such as R. B. Bird, “Transport Phenomena”,Wiley & Sons, New York 1960, or F. P. Incropera, D. P. DeWitt,“Fundamentals of heat and mass Transfer”, Wiley and Sons; 3rd edition,New York, 1996.

For the purposes of the present description, a high thermal diffusivitygas is a gas having a thermal diffusivity of at least 3.0·10⁻⁵ m²/s orhigher, e.g. up to about 2.0·10⁻⁴ m²/s (values at 400° K). Examples ofsuitable high thermal diffusivity gases are oxygen, nitrogen, argon,helium or hydrogen, having a thermal diffusivity at 400° K. of 3.6·10⁻⁵m²/s, 3.7·10⁻⁵ m²/s, 3.8·10⁻⁵ m²/s, 3.0·10⁻⁴ m²/s and 2.3·10⁻⁴ m²/s,respectively.

As the thermal diffusivity of a gas depends, further from its specificthermal diffusivity coefficient, also from the mass fraction of theadded gas, it is preferable to use gases with a higher molecular weight,in order to reduce the volume fraction of added gas (or, alternatively,using the same volume fraction of gas, increase its mass fraction).Oxygen is thus preferred for its higher molecular weight and for itsrelatively high coefficient of thermal diffusivity.

Said high thermal diffusivity gas should preferably be added to the flowof glass precursor material in an amount such that the overall thermaldiffusivity of the so obtained mixture is about 50% higher than thethermal diffusivity of the glass precursor material. In particular, whensilicon tetrachloride is used, the thermal diffusivity of the mixtureshould preferably be higher than about 4.0·10⁻⁶ m²/s at 400° K.Preferably, the thermal diffusivity of the mixture is comprised between4.0·10⁻⁶ m²/s and 5.5·10⁻⁶ m²/s at 400° K.

The high thermal diffusivity gas is preferably admixed in a volumefraction of from about 0.05 to about 0.5 parts with respect to the totalvolume of the mixture, preferably of from about 0.1 to about 0.4 parts,depending also from the thermal diffusivity of the glass precursormaterial (e.g. 2.84·10⁻⁶ m²/s at 400° K. for SiCl₄). The addition of ahigh thermal diffusivity gas to a glass precursor material to be fed toa burner for manufacturing an optical fibre preform is described incopending European Patent Application 00127850.6.

According to a method for manufacturing an optical fibre preform, whichis a further embodiment of the present invention, a combustible gas anda combustion sustaining gas are flown through at least two of theannular channels defined 102 a-108 a. Examples of suitable combustiblegas are hydrogen or hydrocarbons, such as methane. Oxygen is typicallyused as the combustion sustaining gas.

If desired, an inert gas may be flown through some of the annularchannels 102 a-108 a, either alone or admixed with the above combustiblegas or combustion sustaining gas. For instance, an inert gas may beflown through an annular channel disposed between a first annularchannel dedicated to the inlet of a combustible gas and a second annularchannel dedicated to the inlet of a combustion sustaining gas. Thisallows a physical separation of the two flows of combustible gas and ofcombustion sustaining gas, thus displacing the flame away from the tipsof the metal tubes and avoiding possible overheating of the same.Similarly, the flame can be displaced away from the tips of the metaltubes by suitably increasing the inlet speed of the combustible gas andof combustion sustaining gas. Examples of suitable inert gases areargon, helium, nitrogen.

FIG. 6 schematically illustrates a typical overcladding depositionprocess, according to the VAD technique, for embodying the method of thepresent invention. The deposition typically starts onto a glass rod 701of about 20 mm diameter, comprising the core of the preform and a firstportion of the cladding layer, separately manufactured according toconventional techniques. The target preform is rotated about islongitudinal axis and slowly upwardly translated. A lower overcladdingburner 703 deposits a first portion of overcladding layer 702 a, e.g. upto a diameter of about 90-100 mm onto the preform. An upper burner 704then completes the deposition by depositing a second overcladding layer702 b, e.g. increasing the diameter of the deposited soot at about180-200 mm. Typically, the upper burner 704 has increased dimensionswith respect to the lower one, in order to allow the deposition ofhigher amount of silica particles in the time unit.

The so obtained preform is then heated into a furnace and collapsed toobtain a final preform of about 60-80 mm diameter, which is then drawninto an optical fibre according to conventional techniques.

While a burner according to the present invention can advantageously beused in the above process for depositing the overcladding layer of thepreform, in particular the outer overcladding portion (i.e. as burner704), it will be appreciated that such a burner, when suitablydimensioned, can also be used for the deposition of the core and of theinner portion of the cladding.

In a VAD deposition process, it may be advantageous to separate theflame in an inner and an outer flame concentrically disposed. This canbe achieved by interposing a tube of heat resistant material between thetwo flames; said tube may be disposed into the annular housing betweentwo coaxial tubes, said tube of heat resistant material extending for acertain length farther from the tips of the pipes of the inner portionof the burner. The heat resistant material may be suitably chosen amongthose that can be employed for the tube 109 of FIG. 1, as discussedabove.

The use of a tube of heat resistant material is particularly indicatedwhen the coaxial tubes of the burner do not end on the same plane attheir extremity outside the burner; this allows both to physicallyseparate the inner flame from the tubes of the outer section, whichtubes protrude outside the burner more than the tubes of the innersection of the burner and to confine the inner flame. This protects thetubes of the outer section, which can thus be made of metal. Preferably,the flame separating tube extends for a length such to entirely surroundthe reaction zone where the glass precursor material reacts to form theglass particles. Particularly for overcladding burners, the flameseparating tube should preferably extend for at least about 80 mm fromthe tips of the pipes of the inner section of the burner. The length ofthe tube should however preferably not exceed about 150 mm. Preferably,said length is from about 90 to about 130 mm. The use of a flameseparating tube as described above in a multi-flame burner formanufacturing an optical fibre preform is described in copendingEuropean Patent Application 00127851.4.

It may also be preferred to suitably redistribute the flow of formingglass particles before said flow impacts onto the target preform. Tothis purpose, a multi-flame burner with a flame separating tube aspreviously discussed is particularly suitable. In particular, the outletof the flame separating tube may be suitably modified so to increase thedeposition rate of the burner. The modification is such as to confer tothe outlet of the quartz separating tube a cross-section having a majorand a minor axis. It has been observed that the deposition rate can beincreased by increasing the dimensions of the flow of glass particles ina direction substantially perpendicular with respect to the longitudinalaxis of the target preform. To achieve this the terminal portion of theflame separating tube should have a section with a minor and a majoraxis. An elliptical section is particularly suited. In a VAD process formanufacturing an optical fibre preform, the major axis of the section ofthe terminal portion of the flame separating tube should lay on a planesubstantially perpendicular to the longitudinal axis of the growingoptical fibre preform. The technique of redistributing flow of formingglass particles as described above in a multi-flame burner formanufacturing an optical fibre preform is described in copendingEuropean Patent Application 00127849.8.

Example

For this experiment, a burner comprising eight co-axial metal pipes asshown in FIGS. 1 and 5 has been used. The material used for the metalpipes was AISI316L stainless steel. Tubes 101-108 and channels 101 a-103a and 105 a to 108 a of FIG. 5 will be referred to in the presentexample as tubes 1-8 and channels 1a-7a, respectively. A quartz glasstube has been inserted between the third and the fourth metal tube intochannel 104 a of FIG. 5 for providing a flame confinement. The followingtable 1 indicates the relative internal (ID) and outer (OD) diameter ofthe annular channels determined by the metal tubes; for the innermostchannel 1a, having a circular cross section, only the OD has beenreported. The inner section of the burner is formed by tubes 1 to 3 (andcorresponding channels 1a to 3a), while the outer section of the burneris formed by tubes 4 to 8 (and corresponding channels 4a to 7a)

TABLE 1 dimensions of channels Channel no. 1a 2a 3a 4a 5a 6a 7a ID (mm)— 11 21.34 37.6 44.2 55.8 61.1 OD (mm) 7 17.6 24.4 40.2 50.5 58.3 67.55

The internal confining quartz glass tube, having a thickness of about1.5 mm, an inner diameter of 28.4 mm and an outer diameter of 31.4, hasbeen inserted into the annular channel 104 a of FIG. 5 (ID 27.4 mm, OD33.6 mm); said channel was consequently not connected to any feed ofgas. The lower portion of the glass tube has been wrapped with a Teflon®tape up to the outer diameter of the clearance, in order to maintain itin a fixed position.

An outer quartz glass tube (ref. 109 of FIGS. 1 and 5) having athickness of about 2 mm has been further disposed around the outer metaltube 8.

All the metal tubes protruded outside the body of the burner for thesame length (201 mm). The outer quartz tube protruded for about 165 mmfrom the tips of the metal tubes, while the internal quartz tubeprotruded for about 133 mm from the tips of the metal tubes.

All the distribution chambers had a width of 6 mm and a height of 7.5mm.

The diameter of all the ducts for feeding gases into the distributionchambers was of 4 mm.

The reactants employed and their relative flow rate and inlet speed arereported in the following table 2, where the innermost opening of theburner is identified with no. 1a. Silica tetrachloride has been suppliedby vaporizing the liquid material and feeding it at a temperature ofabout 80° C. through the central tube, together with oxygen.

TABLE 2 Reactants and flow rate Channel no. 1a 2a 3a 4a 5a 6a 7aReactant SiCl₄ + O₂ H₂ O₂ Ar H₂ Ar O₂ Flow Rate (slm) 12 + 7 27 65 14160 10 115 Inlet velocity 8.2 3.4 9.9 1.5 5.7 0.7 2.9 (m/s)

The target preform was a rotating quartz tube of about 90 mm diameterand the burner (i.e. the upper end of the outer glass tube of theburner) has been kept at a distance of about 90 mm from the perform,with an inclination of about 12° with respect to the longitudinal axisof the preform.

The preform was translated upwardly at a speed of 168 mm/h and rotatedat about 60 r.p.m.

The deposition was stopped when the preform reached a diameter of about140-150 mm.

By following the above-described procedure, a regular soot depositionwas obtained, without formation of cracks or other defects.

1. A method for feeding a flow of gas to a burner for manufacturing anoptical fibre preform, said burner comprising: a plurality of coaxialtubes, each two adjacent coaxial tubes defining an annular channelbetween themselves; an annular gas distribution chamber at one extremityof at least one of said annular channels and in fluid communicationtherewith, said annular distribution chamber being delimited to theradial direction by an inner and an outer surface; and a feeding duct tofeed a flow of gas into said distribution chamber, said methodcomprising the step of introducing the gas into said distributionchamber in a direction not intersecting the axis of said coaxial tubesand lying on a plane transversal thereto.
 2. The method according toclaim 1, wherein the direction of the gas being introduced into saiddistribution chamber lies on a plane perpendicular to the axis of saidcoaxial tubes.
 3. The method according to claim 2, wherein the directionof the gas being introduced into said distribution chamber does notintersect the inner surface of said distribution chamber.
 4. The methodaccording to claim 3, wherein the direction of the radially outermostportion of said flow of gas, being introduced into said distributionchamber, is substantially tangential to the outer surface of saiddistribution chamber.
 5. The method according to any one of thepreceding claims, further comprising conferring to said flow of gas adirection substantially parallel to the axis of said coaxial tubes alongsaid annular channel.
 6. The method according to claim 5, wherein saiddirection is conferred by flowing said gas through a layer of porousmaterial.
 7. The method according to claim 6, wherein the layer ofporous material is placed between said distribution chamber and saidannular channel. 8-12. (canceled)